1. Field of the Invention
The present invention relates to a technique for determining a knocking state, and in particular, relates to a technique in which intensity values relating to the intensity of vibration of an internal combustion engine are detected, and based on the intensity values larger than a desired value, a knocking state is determined.
2. Description of the Background Art
In general, a knock determination level is set by multiplying an average value of knock sensor signals with a constant K. Such a KCS (Knock Control System) involves a problem that an optimum K value changes depending on manufacturing variations or temporal changes of engines, knock sensors or the like, whereby knock detection will not be able to be performed accurately. In order to solve this problem, there is a technique for determining presence or absence of knock based on the distribution profile of knock sensor signals.
A knock controlling device for an internal combustion engine described in the publication of Japanese Patent Laying-Open No. 01-315649 includes: a knock sensor for detecting knock in the internal combustion engine; a knock intensity value detector for detecting knock intensity values V effective for knock detection from signals of the knock sensor; a knock determiner for determining presence or absence of knock by comparing knock intensity values with a knock determination level; a knock controller for controlling knock control factors such as ignition timing or air fuel ratio based on the determination result; a cumulative % point detector for detecting cumulative % point VP of distribution of almost logarithmically converted values of knock intensity values V each time knock intensity value V is inputted; a standard deviation detector for detecting an actual standard deviation value S of almost logarithmically converted values of the knock intensity values each time knock intensity value V is inputted; and a knock determination level setting unit for setting a knock determination level VKD by VKD=Sn×VP (where n≧2.5) based on cumulative % point VP and actual standard deviation values S. Among detected knock intensity values V, those higher than knock determination level VKD are determined as knock intensity values V caused by knocking. That is, the number (frequency) of knock intensity values higher than knock determination level VKD are determined as the number of times (frequency) that knocking has occurred.
According to the knock controlling device described in this publication, cumulative % point VP of distribution of almost logarithmically converted values of knock intensity values V detected by the knock intensity value detector is detected by the cumulative % point detector each time knock intensity value V is inputted, and the actual standard deviation value of the almost logarithmically converted values of knock intensity values V is detected each time knock intensity value V is inputted. Then, based on cumulative % point VP and the actual standard deviation value, the knock determination level is set by VKD=Sn×VP by the knock determination level setting unit, and the knock determination level and knock intensity values are compared by the knock determiner, whereby presence or absence of knock is determined.
However, in the knock controlling device for an internal combustion engine described in the publication of Japanese Patent Laying-Open No. 01-315649, if a large number of knock intensity values V are detected due to knocking being caused frequently or BGL (Back Ground Level) of the internal combustion engine itself being increased, cumulative % point VP of distribution of almost logarithmic conversion values of knock intensity values V and the actual standard deviation values become larger. In such a case, knock determination level VKD becomes high. Because of it, knocking is not determined as having occurred even though knocking has occurred. Therefore, there is a problem that a knocking occurrence state cannot be determined with high accuracy, including the number of times that knocking has occurred.